Early detection of vegetation physiological stress is beneficial to the environmental and agricultural business community. Plant stresses can be a result of numerous influences including but not limited to drought, chemicals such as herbicides, or biological influences. Early detection can provide an opportunity to reverse the physiological stress or at least identify that stress is present. When unfavorable growth conditions result in plant physiological stress, leaf chlorophyll content typically begins to decrease. Consequently, methods of detecting the content of leaf chlorophyll provide a measure or indication of a level of such stress.
Different approaches to plant stress detection by measuring leaf chlorophyll are available. One such technique which can be used is fluorescence. In the case of fluorescence, incident light is absorbed by leaf pigments. Not all of the absorbed light energy is transferred chemically to be used in photosynthesis. Rather, some of this absorbed energy is re-emitted, or fluoresced, by chlorophyll at far-red, or near-infrared wavelengths. Maximum chlorophyll fluorescence occurs at wavelengths near 690 and 730 nm. For this reason, fluorometers often measure fluorescence with narrow bands centered near 690 or 730 nm. In general, fluorescence in these bands tends to increase with decreased chlorophyll content or increased degree of physiological stress. To measure far-red or near-infrared fluorescence, the leaf is irradiated only with light of much shorter wavelengths (e.g., blue or green light). This insures that any far-red or near-infrared light emanating from the leaf is indeed fluorescence and not merely incident light that has been reflected by the leaf.
A second method of measuring plant chlorophyll content is through the use of transmittance. This technique transmits light through a leaf of a target plant. A percent of light transmitted through the leaf at specific wavelengths is measured. These wavelengths are typically 650 nm and 940 nm. As chlorophyll content changes, the ratio of transmittance at these wavelengths changes. A clear defect in monitoring plant chlorophyll content using this method is the requirement of physical contact with a plant leaf.
Another approach to detecting physiological plant stress by measuring leaf chlorophyll is accomplished by monitoring the reflection of incident light. Reflectance of incident radiation from the leaf interior increases as plant chlorophyll decreases, providing an optical indicator of stress. Reflectance sensitivity analysis has shown that increased reflectance in specific wavebands provides an early and more consistent indication of stress than reflectance at other wavelengths as a result of the absorption properties of chlorophyll. Depending on the severity of stress, this reflectance response can be detected prior to damage symptoms apparent to the unaided eye. Reflectance has been shown to detect decreased chlorophyll content by at least sixteen days prior to visual indications such as leaf color changes. Reflectance measurements are typically made while the plant leaf is exposed to a full incident spectrum from the sun, or an artificial light source. Although some fluoresced energy must also be measured in combination with reflected light, the fluoresced energy is small compared with a greater intensity of reflected light. Further, physical contact with the target plant is not required.
Different techniques are known for conducting reflectance measurements to indicate plant stress. These techniques, however, require extensive field measurements combined with laboratory analysis of the collected measurements. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a portable video imager for detecting plant chlorophyll levels for providing an indication of physiological stress in plants based on reflectance of incident light.